Disposable absorbent articles such as disposable diapers, training pants, adult incontinence garments, and the like are known. Recently, it has become desirable to produce absorbent articles, such as infant diapers, which fit more closely to the body of a wearer. Accordingly, it has become desirable to make such articles smaller and less conspicuous in use while still maintaining a high level of absorbent protection. Specifically, it has become desirable to produce disposable absorbent articles as thin as possible and to make efficient use of super absorbent material by optimally distributing such material in those portions of the article most in need of absorption capacity. As will be discussed further herein, although much effort has been expended by those skilled in the art to achieve such a desirable distribution of the super absorbent material, there is much room for improvement, to which the present invention is addressed.
In order to better understand the problem of distribution of the super absorbent material, first an overview will be presented regarding diaper construction of the prior art.
Conventional disposable diapers generally include an outer cover, a liquid-pervious bodyside liner, and an absorbent material disposed between the outer cover and the bodyside liner. The diaper defines a front portion, a rear portion, and a crotch portion connecting the front and rear portions.
The bodyside liner presents a body-facing surface which is compliant, soft feeling, and non-irritating to the wearer's skin. Further, the bodyside liner may be less hydrophilic than the absorbent material and is sufficiently porous to be liquid pervious, permitting a liquid to readily penetrate through its thickness. It is well known in the art to manufacture the bodyside liner from a wide selection of web materials, such as porous foams, reticulated foams, apertured plastic films, natural fibers (for example, wood or cotton fibers), synthetic fibers (for example, polyester or polypropylene fibers), or a combination of natural and synthetic fibers. The bodyside liner is typically employed to help isolate the wearer's skin from liquids held in the absorbent material. Various woven and nonwoven fabrics can be used for the bodyside liner. For example, the liner may be composed of a meltblown or spunbonded web of polyolefin fibers. The bodyside liner may also be a bonded-carded web composed of natural and synthetic fibers. For the purposes of the present description, the term "nonwoven web" shall mean a web of material which is formed without the aid of a textile weaving or knitting process. The bodyside liner may be composed of a substantially hydrophobic material, and the hydrophobic material may optionally be treated with a surfactant or otherwise processed to impart a desired level of wettability and hydrophilicity.
The outer cover generally comprises a material which is configured to be substantially impermeable to liquids. For example, a typical outer cover can be manufactured from a thin plastic film or other flexible liquid-impermeable material. The outer cover generally prevents the exudates contained in the absorbent material from wetting articles, such as bed sheets and overgarments, which contact the diaper. The size of the outer cover is typically determined by the size of the absorbent material and the exact diaper design selected. The outer cover, for example, may be generally T-shaped, generally I-shaped, or may have a modified hourglass shape, and may extend beyond the terminal edges of the absorbent material by a selected distance.
The absorbent material is adapted to absorb body exudates. The absorbent material may comprise a single, integral piece of material or, alternatively, may comprise a plurality of individual, separate pieces of material which are operably assembled together. The absorbent material may be manufactured in a wide variety of sizes and shapes (for example, rectangular, trapezoidal, T-shaped, I-shaped, hourglass shaped, etc.), and from a wide variety of materials. The size and the absorbent capacity of the absorbent material are selected to be compatible with the size of the intended wearer and the liquid loading imparted by the intended use of the absorbent article. The size and the absorbent capacity of the absorbent material can be varied to accommodate wearers ranging from infants through adults.
Various types of wettable hydrophilic fibrous material can be used to form the absorbent material. One commonly used material is comminuted wood pulp, generally referred to in the art as airfelt. Examples of suitable fibers include naturally occurring organic fibers composed of intrinsically wettable material, such as cellulosic fibers; synthetic fibers composed of cellulose or cellulose derivatives, such as rayon fibers; inorganic fibers composed of inherently wettable material, such as glass fibers; synthetic fibers made from inherently wettable thermoplastic polymers, such as particular polyester or polyamide fibers; and synthetic fibers composed of a non-wettable thermoplastic polymer, such as polypropylene fibers, which have been hydrophilized by appropriate means. The fibers may be hydrophilized, for example, by treatment with silica, treatment with a material which has a suitable hydrophilic moiety and is not readily removable from the fiber, or by sheathing the non-wettable, hydrophobic fiber with a hydrophilic polymer during or after the formation of the fiber. For the purposes of the present invention, it is contemplated that selected blends of the various type of fibers mentioned above may also be employed. As used herein, the term "hydrophilic" describes fibers or the surfaces of fibers, which are wetted by the aqueous liquids in contact with the fibers. The degree of wetting of the materials can, in turn, be described in terms of the contact angles and the surface tensions of the liquids and materials involved. Equipment and techniques suitable for measuring the wettability of particular fiber materials or blends of fiber materials can be provided by a Cahn SFA-222 surface force analyzer system. When measured with this system, fibers having contact angles less than 90.degree. are designated "wettable", while fibers having contact angles greater than 90.degree. are designated "non-wettable".
The above fibrous materials shall be referred to herein collectively and interchangeably by the term "pulp". In addition to the fibrous materials described above, the absorbent material may further comprise a high-absorbency material such as those known in the art as "superabsorbents", which shall be referred to herein collectively and interchangeably by the term "SAP", standing for superabsorbent polymer, and it should be clear that the term SAP is not restricted just to synthetic materials. High-absorbency materials can be natural, synthetic, and modified natural polymers and materials. In addition, the high-absorbency materials can be inorganic materials such as silica gels, or organic compounds such as crosslinked polymers. The term "crosslinked" refers to any means for effectively rendering normal water-soluble materials substantially water insoluble but swellable. Such means can include, for example, physical entanglement, crystalline domains, covalent bonds, ionic complexes and associations, hydrophilic associations such as hydrogen bonding, and hydrophobic associations or van der Waals forces.
Examples of synthetic high-absorbency materials include polymeric materials, such as alkali metal and ammonium salts of poly(acrylic acid) and poly(methacrylic acid), poly(acrylamides), poly(vinyl ethers), maleic anhydride copolymers with vinyl ethers and alpha-olefins, poly(vinyl pyrrolidone), poly(vinyl morpholinone), poly(vinyl alcohol) and mixtures and copolymers thereof. Further polymers suitable for use in the absorbent material include natural and modified natural polymers, such as hydrolyzed acrylonitrile-grafted starch, acrylic acid grafted starch, methyl cellulose, carboxymethyl cellulose, hydroxypropyl cellulose, carboxymethyl starch, and the natural gums, such as alginates, xanthan gum, locust bean gum, and the like. Mixtures of natural and wholly or partially synthetic absorbent polymer can also be useful in the present invention. Other suitable high-absorbency materials are disclosed by Assarson et al. in U.S. Pat. No. 3,902,236 issued Aug. 26, 1975. Processes for preparing synthetic: high-absorbency materials are disclosed in U.S. Pat. No. 4,076,663 issued Feb. 28, 1978, to Masuda et al. and U.S. Pat. No. 4,286,082 issued Aug. 25, 1981, to Tsubakimoto et al. The disclosures of these aforementioned patents are incorporated herein by reference.
It has been found, that in actual use, the front and crotch portions must generally absorb the vast majority of exudates, whereas the rear portion generally absorbs a very minimal amount of exudates. Inasmuch as the SAP is considerably more expensive than the pulp, it is desirable to make efficient use of the SAP. Therefore, the optimum distribution of the absorbent material calls for concentrating SAP in the front and crotch portions and restricting the SAP to a minimal amount in the rear portion.
High speed machinery is used for mass production of diapers in the current art. A rotating drum is prepared with molds on its outer circumference. Each mold is a kind of pocket for receiving therein pulp, and has a screen backing facing the inner cavity of the drum. A powerful centrifugal blower is arranged to suck air from outside the drum through the molds and the screens into the inner cavity of the drum. The sucking force of the blower forces the pulp to be forcefully packed into the molds as the drum turns. SAP is also introduced into the molds during rotation of the drum, such that the SAP intermingles with the pulp to form the absorbent material core of the diaper. The absorbent material core just formed exits the drum to a conveyor belt for assembling therewith the bodyside liner, outer core and other portions of the complete diaper.
Apparatus is known for controlled introduction of the SAP into the drum in an effort to optimize its placement in the absorbent material core. An example of such apparatus is described in applicant/assignee's U.S. Pat. No. 5,514,324, the disclosure of which is incorporated herein by reference. The apparatus of the aforementioned patent is capable of achieving a distribution of about 88% of the total SAP in the front and crotch portions and 12% in the rear portion.
Very recently ready-made absorbent core materials have been developed, known in the art as air-laid materials. Air-laid materials may be supplied as a pre-formed roll of just pulp or a combination of pulp and SAP in any proportion desired. This is a new development which has not yet seen widespread commercial use. An advantage of the pre-formed air-laid rolls is that they eliminate noisy machines which are normally used in the art to grind or otherwise pulverize the pulp before combining with the SAP. The air-laid rolls are simply fed into the high-speed manufacturing lines used in the art. As soon as one roll is done, a fresh roll is fed into the manufacturing line. Thus, in order to make manufacturing more cost-efficient, it is desirable to decrease the frequency of having to feed fresh air-laid rolls. Certainly this frequency can be decreased by using larger rolls. However, there is a limit to the amount of one-layer diapers which may be manufactured from one air-laid roll, because as the diameter of the roll increases, the roll becomes mechanically unstable or starts to buckle. This limiting diameter is a function of material properties and width of roll. Therefore, the limiting width places a practical limit on the number of diapers manufacturable from such a roll.
It should also be noted that the above 88% to 12% SAP distribution of U.S. Pat. No. 5,514,324 is not the ideal distribution for air-laid rolls. For an air-laid roll, the idea distribution is considered to be 95% of the total SAP in the front and crotch portions and 5% in the rear portion. For example, for an air-laid roll which is pre-manufactured with 5% of the SAP in the rear portion, it is desirable to place all of the SAP in the front and crotch portions. However, such a distribution is beyond the limits of the known technology of the art.